A field effect transistor using a nitride-based Group III-V semiconductor epitaxial substrate (hereinafter referred to as “GaN-FET”) is a field effect transistor constituted such that a GaN layer drives as a channel layer, and it is an element which rapidly becomes widely noticed of late from that the GaN-FET has high voltage resistance, high heat resistance and small environmental load of constituent materials as compared with the conventional FET having a structure that an epitaxial semiconductor crystal layer such as GaAs, AlGaAs, InGaAs, InGaP and AlInGaP is used as a channel layer.
The GaN-FET has various types in view of the structure of an operating layer. In particular, the GaN-FET of the type that a two-dimensional electron gas (hereinafter referred to as “2DEG”) induced in the neighborhood of the interface between nitride semiconductor materials having different lattice constant is driven as a channel is called GaN-HEMT where HEMT refers to High Electron Mobility Transistor. The GaN-HEMT has the characteristics of excellent frequency characteristics and high power density, in addition to the above characteristics, and is strongly expected to put into practical use.
The GaN-HEMT is prepared by laminating epitaxial crystals on a ground substrate by, for example, an electron beam epitaxial growth method (hereinafter referred to as “MBE method”) or a metalorganic vapor phase growth method (hereinafter referred to as “MOVPE method”), and processing the laminate into the desired device shape by photolithography. The example of structure of the GaN-HEMT can refer to, for example, literature references.
In the case that MOVPE method is used as a lamination method of semiconductor crystals for the production of an epitaxial substrate for GaN-HEMT, an epitaxial substrate having a given layer structure can be obtained by heating a ground substrate such as monocrystal sapphire, monocrystal silicon carbide (hereinafter referred to as “SiC”) or monocrystal silicon in a reactor, successively feeding trimethylgallium, trimethylaluminum, ammonia and dopant gas as raw material gases to the reactor, thermally decomposing those gases on the substrate, and successively depositing AlN buffer layer, undoped GaN layer (hereinafter referred to as “ud-GaN”), undoped AlGaN (hereinafter referred to as “ud-AlGaN”) and n-type AlGaN (hereinafter referred to as “n-AlGaN”) on the substrate.
In the case of the above-illustrated layer structure, 2DEG is formed at the interface between ud-AlGaN layer and ud-GaN layer, and this forms a channel and operates as FET. AlN buffer layer and a lower layer side not containing a channel of ud-GaN layer (hereinafter referred to as “ud-GaN buffer layer”) are introduced to relax mismatching at the epitaxial growth between a ground substrate and a channel forming layer due to difference in lattice constant and difference in coefficient of thermal expansion and to form a channel layer having less defect. The ground substrate such as monocrystal sapphire, SiC and monocrystal silicon described above each has large difference in lattice constant and large difference in thermal expansion to GaN crystals. Therefore, in the production of FET using those substrates, the ud-GaN buffer layer is generally grown in large thickness (usually 1 μm or more) such that the ud-GaN buffer layer exhibits sufficient buffer effect. For such a buffer layer, a reference is, for example, GROUP III NITRIDE SEMICONDUCTOR, written and edited by Isamu Akasaki, p 157 (1999), Baifukan.
For the purpose of the general discussion, a layer having a role of AlN buffer layer in GROUP III NITRIDE SEMICONDUCTOR, written and edited by Isamu Akasaki, p. 157 and p. 291 (1999), Baifukan is called a first buffer layer, and a layer having a role of ud-GaN buffer layer is called a second buffer layer, in the following description. It is ideal in the operation of GaN-HEMT that current injected from a source electrode flows into a drain electrode through only a channel part, and it is not preferred that current flows into the first buffer layer or the second buffer layer. If current flowed into the first buffer layer or the second buffer layer, even though a channel is electrically depleted by applying voltage to a gate electrode, current flowing between the source electrode and the drain electrode is not completely shielded. This leads to the problems of deterioration of pinch-off characteristics and increase in drain leakage. Furthermore, the unnecessary current component has low mobility different from 2DEG. As a result, in the case of operating the gate electrode with high frequency voltage, adverse influence such as frequency dispersion is involved. Additionally, the unfavorable unnecessary current also flows into adjacent other elements, causing the interference such as fluctuation of threshold voltage of the adjacent elements.
To avoid the above-described various problems caused in FET, it is effective to insulate the first buffer layer, the second buffer layer or a part thereof, that is, to increase resistance to an extent such that only current capable of ignoring influence as compared with a size of channel current flows. When high resistivity layer is formed in this part, electrons flowing from the source electrode is shielded by the layer, and do not exude in the lower part than the layer. As a result, FET can easily achieve pinch-off. In general, nitride-based Group III-V monocrystal has extremely high stability chemically and physically, and deep element isolation processing as reaching the substrate is extremely difficult. However, in the case of introducing a high resistivity layer as above, interference to the adjacent elements can easily be prevented if only element isolation processing is carried out to only the depth up to the high resistivity layer.
However, it is not easy to epitaxially grow high resistivity nitride Group III-V monocrystal. Nitride Group III-V monocrystal epitaxially grown under the general conditions tends to show high n-type conductivity even though impurities are not intentionally added. The reason is based on the following interpretations. Because the nitride Group III-V monocrystal is grown at relatively high temperature, nitrogen atom having high dissociation pressure is easily withdrawn from crystals, and the voids generate free electrons. In a gas phase growth method, the nitride Group III-V monocrystal has shallow donor level by the incorporation of oxygen which is impurity easily incorporated from the atmosphere, and free electrons are easily generated, thereby giving 2n-type conductivity. For the cause of the n-type conductivity shown by GaN crystal, a reference is, for example, Chris G. Van de Walle, Catherine Stampfl, J. Crystal Growth 189/190 (1998), 505-510.
There is also the reason due to laminate structure of crystal. That is, the nitride Group III-V monocrystal has large difference in lattice constant to the ground substrate as described above. Various crystal defects are present in crystal. The defect has a level inherent in defect species. Some are easily ionized, thereby imparting conductivity to crystal.
One measure to make the epitaxial crystal semiconductor have high resistivity is a method of introducing charge compensation impurity into crystal. The charge compensation impurity means impurity which forms deep level accepting electron in forbidden band. Electrons flowing into the layer containing the impurity are rapidly captured by the level and restricted. Therefore, a semiconductor layer having a large amount of the impurity doped therein acts as an extremely high resistivity layer. Realization of a high resistivity layer by the measure and the effect in the case of applying to FET are conventionally known. For example, in a gallium arsenide semiconductor, an example of forming a deep acceptor level by doping oxygen in AlGaAs semiconductor crystal epitaxially grown, thereby realizing a high resistivity epitaxial layer is provided by Sasajima Y., Fukuhara N., Hata M., Maeda T., and Okushi H., Power Semiconductor Materials and Devices Symposium, 425-430 (1997). Furthermore, an example of applying the epitaxial layer to a buffer layer of FET, thereby obtaining good pinch-off characteristics is provided by Japanese Patent No. 2560562.
This measure can be expected to be effective even in a gallium nitride-based semiconductor, and various investigations have been already made and reported. For example, D. S. Katzer, D. F. Storm, S. C. Binari, J. A. Roussos and B. V. Shanabrook, J. Crystal Growth 251 (2003) 481-486 reports GaN-HEMT using a buffer layer obtained by doping beryllium (Be) in GaN crystal by MBE method. According to the report, it is reported that GaN layer reduced leakage current in lateral direction in triple digits by doping beryllium, and pinch-off characteristics were remarkably improved in FET using this layer as a buffer layer.
Further, J. B. Webb, H. Tang, S. Rolfe, and J. A. Bardwell, Appl. Phys. Lett., 75 (1999) 953 reports an example that hetero structure of AlGaN/GaN was epitaxially grown on a buffer layer obtained by doping carbon (C) in GaN crystal by MBE. According to this report, it is reported that an extremely high resistivity GaN buffer layer having resistivity of 106 Ωcm was obtained by doping carbon, and AlGaN/GaN hetero structure-induced 2DEG laminated thereon had good characteristics that mobility is 1,200 cm2/V/S.
According to those reports, it is reported that where those impurities are doped in GaN layer and the GaN layer is applied to FET, certain effect can be expected in the improvement of characteristics of FET.
However, the above-described prior arts have the following problems on production. It is known that beryllium has extremely strong toxicity, and load to safety and environment is extremely large. Thus, the application of beryllium to production is not always practical. Carbon has an atomic radius extremely greater than that of gallium atom (hereinafter referred to as “Ga”) and nitrogen atom (hereinafter referred to as “N”) constituting gallium nitride crystal (Ga: 0.76 angstrom, N: 1.57 angstroms, C: 2.46 angstroms). Where carbon is doped in crystal in high concentration, lattice distance of crystal is disturbed, resulting in deterioration of crystallizability.
In MOVPE method, carbon tetrabromide and carbon tetrachloride are generally used as raw material gas of carbon. Those have bromine and chlorine in the molecule. Therefore, where those are introduced into a reactor, bromine gas or chlorine gas is generated. This gas etches an epitaxial layer, resulting in deterioration of crystallizability. In the growth of GaN crystal, tetramethyl gallium and tetraethyl gallium are generally used as raw material gas of gallium. In the reaction in which those are crystallized as Ga, it is known that those simultaneously release C, and the C is incorporated into an epitaxial layer. The incorporation amount quickly changes by growth rate, growth pressure and the like as parameters of vapor deposition. That is, it is difficult in MOVPE method to control C concentration with good accuracy by only controlling flow rate of a C precursor into a reactor, though the other doping materials is not difficult to be controlled by the flow rate.
Even though the production can be performed by avoiding the above problems on production, where compensation impurities are present in a layer, other disadvantages may be posed in FET characteristics. That is, the compensation impurities capture electrons in the inherent normal state to immobilize. Therefore, when the compensation impurities are dispersed in the vicinity of a channel layer, this affects the running itself of channel electrons related to FET operation. The influence is developed as turbulence of waveform which is unfavorable for FET, such as generation of kink 400 at 1-5 characteristics.